Plastic materials used in the manufacture of powder coatings are classified broadly as either thermosetting or thermoplastic. In the application of thermoplastic powder coatings, heat is applied to the coating on the substrate to melt the particles of the powder coating and thereby permit the particles to flow together and form a smooth coating.
Thermosetting coatings, when compared to coatings derived from thermoplastic compositions, generally are tougher, more resistant to solvents and detergents, have better adhesion to metal substrates and do not soften when exposed to elevated temperatures. However, the curing of thermosetting coatings has created problems in obtaining coatings which have, in addition to the above-stated desirable characteristics, good smoothness and flexibility. Coatings prepared from thermosetting powder compositions, upon the application of heat, may cure or set prior to forming a smooth coating, resulting in a relatively rough finish referred to as an "orange peel" surface. Such a coating surface or finish lacks the gloss and luster of coatings typically obtained from thermoplastic compositions. The "orange peel" surface problem has caused thermosetting coatings to be applied from organic solvent systems which are inherently undesirable because of the environmental and safety problems that may be occasioned by the evaporation of the solvent system. Solvent-based coating compositions also suffer from the disadvantage of relatively poor percent utilization, i.e., in some modes of application, only 60 percent or less of the solvent-based coating composition being applied contacts the article or substrate being coated. Thus, a substantial portion of solvent based coatings can be wasted since that portion which does not contact the article or substrate being coated obviously cannot be reclaimed.
Of particular interest are the acrylic copolymers containing glycidyl functional groups, such as PD 7610 containing glycidyl methacrylate (Mitsui Toatsu), which are crosslinked with dicarboxylic acids to produce highly weatherable powder coatings. Linear aliphatic dicarboxylic acids are known in the art and are preferred as crosslinking agents in such systems. Examples of such crosslinkers include those described by the general formula HOOC--(CH.sub.2).sub.n --COOH, wherein n is an integer of 2 to 20. The most highly recommended dicarboxylic acid is dodecanedioic acid (DODA). However, the surface hardness of glycidyl acrylic-based coatings cured with DODA possess a relatively low (F--H) pencil hardness and only a little increase in hardness was obtained by the use of lower carbon number acids (See GCA Product Information Technical Bulletin 607).